Rate control for tuning fork oscillators



April 18,1944. M. s. mama, JR 2,3 6, 8

RATE CONTRCIL FOR IEQRK OSCILLATOR-5 Fxiled Aug. 25,, 1,942

I FORK AMPLIFIER FORK AMPLIFIER Fig.5.

DECREASE INCREASE RATE RATE Fig.2.

| I ER 8 44 i P6. E9 Inventor:

Milton S. Mead J11,

y His Attorney.

SECONDS PER-DAY Patented Apr. 18, 1944 RATE CONTROL FOR TUNING FORKOSCILLATORS Milton S. Mead, Jr., Hempstead, N. Y., assignmto GeneralElectric Company, a corporation of New York Application August 25, 1942,Serial N0. 456,001

'7 Claims.

My invention relates to oscillation generators, and more particularly tooscillation generators employing an electron discharge control deviceand including a frequency determining system of the mechanicallyvibratory type. It is an object of my invention to provide a control forsuch an oscillator which allows a wide range of adjustment of the rateof mechanical vibration of the frequency determining system.

In applications where it is necessary to maintain the output frequencyof an oscillator substantially constant over long periods of time, as inclock supervisory systems, for example, oscillators employing amechanically vibratory element, for example a tuning fork, have beenfound particularly suitable. Such oscillators usually include drive andpick-up coils associated with the tuning fork, which coils are coupledrespectively to the output and input circuits of an electron dischargeamplifier and are so arranged that vibration of the element controls theamplifier to produce sustained oscillations in the output circuitthereof.

It is an object of my invention to provide in an oscillation generatorof the above described type means for controlling the frequency ofvibration of the vibrating element which, while allowing a wide range ofadjustment of the rate of vibration, maintains a constant amplitude ofvibration of the vibrating element and consequently constant outputvoltage from the electron discharge amplifier.

A further object of my invention is to provide means for varying thefrequency of vibration of a vibratory element by shifting the phaseangle between the current in the driving coil and the vibrations of theelement in such a manner that the change in frequency varies linearlywith the position of a manual control member.

Another object of my invention is to provide a tuning fork oscillatorhaving an adjustable rate of vibration which can be easily produced inlarge quantities and which includes means for readily adjusting therange of rate control for such production units.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawing, in

- which Fig. 1 illustrates one embodiment of my in vention; Fig. 2 showsvectorially certain voltage relationships pertaining thereto; Fig. 3shows an the discharge device. device 3 is similar in all respects todevice I, with the exception that the single diode elecexample of thescale possible in the rate control device; and Figs. 4 and 5 illustrateschematically modifications of my improved rate control.

Referring to Fig. 1 of thedrawing, I have shown my improved oscillationgenerator as comprising an amplifier, including an electron dischargedevice I, which is controlled by a vibratory element 2 to generateelectrical oscillations having a frequency determined by the frequencyof vibration of the element 2. The vibratory element 2 is shown as atuning fork, although it will be understood that other forms ofmechanical vibratory elements may be employed Without modification ofthe circuit illustrated. The output of the device I is impressed of thesingle-diode pentode type and includes an anode 5, a cathode 6, controlelectrodes '1, 8 and 9 and a diode electrode I0, control electrode 9being directly connected to the cathode ,6 and functioning as the usualsuppresser grid to prevent secondary electron emission within Theelectron discharge trode is omitted.

The input circuit to the amplifier I includes asecondary ll of acoupling transformer I2 having its primary I3 connected to a pick-upcoil I4 wound on the field structure of the tuning fork 2. For a purposeto be explained more fully later, the input circuit is connected acrossa phase shifting circuit, comprising resistance I5 and capacitor I6, thevoltage across the capaci- -tor I6 being impressed between the controlelectrode 1 and the cathode B, of the discharge device I through analternating current, by-pass condenser II.

The output circuit of the amplifier I includes the primary Winding I8 ofa coupling transformer having its secondary winding I9 connected througha rate control .network 20, the

nature and function of which will be described in detail later, toexcite a fork drive coil 2|. The

.winding I8 is connected between the anode 5 and cathode 6 of the deviceI in series with a source of high voltagedirect current potential 22 anda resistance 23. .A condenser 24 is connected in shunt with the windingI8 and functions to tune the inductance of the winding to form anoscillatory circuit 25 having a resonant frequency equal to the normaloperating frequency which the apparatus is designed to produce.

The potential developed across the terminal of the oscillatory circuit25 is impressed upon a resistance 25 connected to the oscillatorycircuit by a pair of alternating current by-pass condensers 2'! and 28included in the input circuit of device 3. Resistance 26 is connectedbetween the control electrode and cathode of the discharge device 3through a by-pass condenser A resistance 38, connected at one end to thenegative side of the voltage source 22, serves to bias the controlelectrode ofv the device 3 negative with respect to the cathode thereof.

Th output circuit of the electron discharge device 3 includes the leads4 connected respectively to the anode and cathode of the deviceelectrode of device 3 through the voltage droping resistor 33. Thisresistor serves to-maintain the screen electrode at a positive potentialslightly lower than the positive potential of the anode of device 3. Insimilar manner a biasing potential is impressed on screen electrode 8 ofdevice 1 through thecircuit lead' which includes a voltage droppingresistor 34.

The vibratory frequency determining unit for controlling -the amplifiersystem described above includes the tuning fork 2 and a field structurecomprising the core members 35, 36, upon which the drive and pick-upcoils respectively are wound, and the permanent magnet member 31. Sincethis structure forms no part of my present invention, it will not bedescribed in detail; instead, a description of this structure may befound in my UnitedStates Letters Patent No. 2,147,492, granted February14, 1939, and assigned to the same assignee-as the present invention.The intensity of the unidirectional flux flowing between members 35 and35, and in turn the negative compliance or stiffness of the tines offork .2 may be adjusted by means of winding 38 on member 31 which isconnected in series with thebattery 39 and the switch.

As has been previously-noted herein, my present invention is primarilyconcerned with the problem of providing means by which the rate ofmechanical vibration of thetines of fork 2 may be varied over a widerange. For this purpose the rate control network 20 is provided acrossthe secondary winding l9 supplying the fork drive coilil. This networkcomprises capacitor 4| and resistance 42 connected in series acrosswinding l9 and having a common point 43 to which is attached oneterminal of the drive coil 2| A fixed resistance 44 connected in shuntwith the above mentioned'series circuit is provided with a variablepoint 45 to which theother terminal of drive coil 2| is connectedthrough secondary winding of transformer 41, theprimary winding 48 oftransformer 41 being connected in series withpick-up coil l4 and theprimary winding l3 of transformer I2.

Operation of the above described oscillation generator may be initiatedby impressing a current impulse on the drive coil 2|. Such an impulsechanges the path of the flux, flowing betines of fork 2 by attracting orrepelling these tines in accordance with the polarity and magnitude ofsaid current impulse. Such movement of the tines of fork 2 alters themagnetic flux linking pick-up coil l4, generating in the pick-up coil anelectromotive force which is a function of the current flowing in drivecoil 2|. This electromotive force is impressed on the control electrode1 of amplifier I through the coupling transformer l2 and phase shiftingnetwork l5, l6, where a phase shift of 90 is obtained, and

. causes an impulse of energy to flow in the output circuit of device i.A portion of this impulse of energy developed in the output circuit isdelivered through the secondary winding I9 to the rate control circuit23. When the variable contact 45 is adjusted to the mid point on theresistor 44, rate control network 29 produces a phase shift equal andopposite to that obtained in the phase shifting network l5 and I5. Undersuch conditions a current impulse flows in drive coil 2| in a directionopposite to the current flow of the initial current impulse. Thisproduces an opposite movement of the tines of fork 2 causing a secondvoltage impulse to be generated in pick-up coil l4 having a signopposite to that caused by the first movement of the tines. This secondimpulse of voltage exerts a controlling effect on the device I such thatthe instantaneous current flowing in the output circuit thereof isreversed in direction for the duration of the impulse. The interactionof fork vibration and amplifier ultimately reaches a condition ofequilibrium in which the fork is vibrated at a substantially constantfrequency determined by the amplitude of the alternating current in thedrive coil 2|, the intensity of the unidirectional flux between members35 and 36 and the natural period of vibration of the fork.

By adjustment of the position of variable contact 45, the phase betweenthe voltage applied across the terminals of drive coil 2| and thevibrations of the fork may be altered so that the driving force exertedby the drive coil either leads or lags the fork vibrations. When thedriving current is in phase with the vibrations of the fork, and hencewith the pick-up voltage, the fork operates at its natural frequency. Ifthe driving current is made to lag behind the vibrations of the fork, itreduces this frequency to a value below the natural frequency. On theother hand, if the driving current leads the vibrations of the fork, therate of vibration is increased.

The manner in which my control acts to ine crease or decrease the rateof vibration of the ,element 2 may be explained by reference to Fig. 2.

In Fig. 2 if the vector E represents the voltage across the terminals ofthe winding |9, voltages across resistance 42 and capacitor4| are shownrespectively by vectors ER andEc. The current flowing through the seriescircuit of resistor 42 and capacitor 4| is shown bythe vector I. It is,of course, apparent that the vo1tage:E represents likewise the voltageexisting across resistance 44. The vector Eb, therefore, represents thevoltage applied across the drive coil 2| when the variable point 45 isadjusted to the mid position of resistance 4.4. Under such conditions,the circuit constants are adjusted so that the driving-force producedbythe current flowing :in drive-coil 2| is in phase with the vibrationsofthe fork. When the variable contact 45 is moved to some other position,.such as 45", the voltage applied=across theterminals 43, '45 of thedrive coil circuit is tween the core, members '35 and 36 through 1-,he

represented by the'vector-E'D. Under these conditions the driving forcelags the vibrations of the fork and reduces the output frequency of thesystem. The increased driving current secured when the control is offcenter position is automatically the amount required to furnish theincreased driving force necessary to cause the fork to vibrate with thesame amplitude at the new phase angle, which the fork must assume tomatch the phase shift through the amplifier.

As a result of this constant amplitud of vibra-' tion of the fork, themagnitude of the voltage generated in the pick-up coil I4 remainsconstant, resulting in a constant output voltage for the amplifiers Iand 3. The out-of-phase component of the voltage E'n is effective toreduce or increase the frequency of the voltage appearing in the pick-upcoil I4 with resultant variation in the frequency of a voltage acrossthe terminals 44. Moreover, the vector diagram of Fig. 2 shows that themagnitude of this out-ofphase component of the voltage of E'p varieslinearly with the adjustment of the contact 45 from the mid point ofresistance 44.

Fig. 3 shows an example of the scale which may be used for the ratecontrol when a well known type of potentiometer is used for the variableresistance 44. The actual value of increase or decrease in frequency isa function of the decrement of fork 2. If the fork has a very lowdecrement, it can vibrate with a large phase angle between the drivingforce and the fork velocity without experiencing but a slight change infrequency.

In order to neutralize the effect of voltages generated in the drive andpick-up coils due to the inductive coupling between these two coils, thecoils are both loosely coupled and transformer 41 is provided, with itswindings 46 and 48 connected respectively in the drive and pick-upcircuits, to introduce in either of the two circuits a potential whichis opposite in phase to the potential generated by the coupling of thecoil windings and equal in magnitude to the voltage produced by suchcoupling.

Also, in order to maintain as nearly constant as possible the magnitudeof the output current of the amplifier I, a circuit is provided forsupplying a biasing potential to control electrode 1 ofdevice I whichvaries in accordance with the intensity of the output current fiowingbetween the anode 5 and the cathode 6. This automatic biasing circuit,comprising the diode formed by the element Ill and the cathode 6,coupling capacitor 49 and resistances 50 and 5|, functions to maintainsubstantially constant the anode current of device I by utilizing theunidirectional current flowing between element In and cathode 6,resulting from the alternating potential impressed thereacross throughcapacitor 49, to develop a unidirectional potential across resistor 50.This unidirectional potential is impressed on control electrode 1through resistance 5i and winding II of transformer I2 and varies thebiasing potential impressed on the control electrode 1 with, and inproportion to, the magnitude of the current flowing in the anode circuitof the amplifier.

In order to provide a constant potential for the screen electrode 8 ofdevice I, an electric discharge device 52 is connected between theelectrode 8 and the cathode 6. Since the screen electrode 8 is connectedto the source of potential 22 through resistance 34, the device 52,which may comprise an ordinary glow discharge tube having a pair ofspaced electrodes positioned within a container filled with an ionizablemedium, such as neon, acts in a well known manner to vary the currentflow in direct proportion with variations of the potential source 22,thus maintaining the potential of the grid 8 with respect to the cathode6 at a constant value. In this manner the magnitude of the currentsupplied to the drive coil 2| is maintained substantially constant.Switch 53 is provided in order to supply a high voltage impulse forstarting tube 52.

In the rate control for a tuning fork-oscillator illustrated in Fig. 1,a wide range of adjustment may be secured by varying the position ofcontactv 45 between the extreme limits of resistor 44, a phase shift ofin operation being secured with constant fork amplitude.

In Fig. 4 is shown one modification of the control illustrated inFig. 1. In this figure, parts corresponding to Fig. 1 are identified bycorresponding reference characters. Inthis modification fixed resistors54 and 55, of equal ohmic value, are connected in series with resistor44, decreasing the rangeof rate variation.

In the modification illustrated in Fig. 5, I have shown how my ratecontrol may be connected between the pick-up coil and the input to thefork amplifier when operating conditions make sucha change desirable.

It will thus be seen that my improved oscillation generator providesmeans whereby a wide range of adjustment of the rate of vibration of amechanical vibratory element may be secured while the output voltagefrom the vibration amplifier is held constant for all settings of therate control. Moreover, my control is characterized by the linearrelationship between the change in frequency and the change in thesetting of a potentimeter.

While I have shown particular embodiments of my invention, it will ofcourse be understood that I do not wish to be limited thereto sincevarious modifications may be made, and I contemplate by the appendedclaims to cover any such modifications as fall within the true spiritand scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination, a vibratory element, means associated therewith forgenerating a voltage having an intensity determined by the amplitude ofvibration of said element anda frequency determined by the frequency ofvibration of said element, an output circuit connected to said! voltagegenerating means, means including a drive coil coupled to said outputcircuit for supplying a driving force to said element, and means forvarying the phase between said driving force and said element, saidphase varying means comprising a resistance and capacitor connected inseries across said output circuit, a second resistance connected inshunt therewith, and means for connecting said drive coil between thecommon point of said resistance and capacitor and a variable point onsaid second resistance.

2. In combination, an oscillation generator comprising a vibratoryelement, an electron discharge device having an anode, a cathode, and acontrol electrode, input and output circuits connected to said device,means including a drive coil coupled to said output circuit forsupplying a driving force to vibrate said element, means including apick-up coil for controlling the frequency of current in said outputcircuit in accordance with the frequency of vibration of said element,and means for varying the frequency of vibration of said element, saidlast-named means comprising a phase-shifting circuit connected acrosssaid output circuit, and means for adjustably connecting said drive coilacross said phaseshifting circuit, whereby the phase angle between saiddriving force and said element is varied.

3. In combination, an oscillation generator comprising avibratoryelement, an electron discharge device having an anode, acathode, and a control electrode, input and output circuits connected tosaid device, means including a drive coil coupled to said output circuitfor supplying a driving force to vibrate said element, means including apick-up coil for controlling the frequency of current in said outputcircuit in accordance with the frequency of vibration of said element,and means for varying the frequency of vibration of said element, saidlast-named means comprising a capacitor and a first resistance connectedin series across said output circuit, said capacitor and firstresistance having a common point, a second resistance connected in shuntto said output circuit, and means for connecting said drive coil betweensaid common point and a variable point on said second resistance,whereby the phase angle between said driving force and said element maybe varied.

4. In combination, a vibratory element, means associated therewith forgenerating a voltage having an intensity determined by the amplitude ofvibration of said element and a frequency determined by the frequency ofsaid vibration, and means for varying the frequency of said vibrationwhile maintaining the amplitude of said vibration constant, said meanscomprising an output circuit connected to said voltage generating means,phase-shifting 'means connected across said output circuit, and meansincluding a driving coil connected to said phase-shifting means forsupplying a driving force to said element, the phase angle between saidforce and said vibration being adjustable.

5. In combination, a vibratory element, means associated therewith forgenerating a voltage having an intensity determined by the amplitude ofvibration of said element and a frequency determined by the frequency ofsaid vibration, an amplifier having input and output circuits, meansincluding a phase-shifting circuit connecting said voltage generatingmeans to said input circuit, and means for varying the frequency of saidvibration while maintaining the amplitude of such vibration constant,said means comprising phaseshifting means connected across said outputcircuit, and means includinga driving coil connected to saidphase-shifting means for supplying a driving force to said element, thephase angle between said force and said vibration being adjustable.

6. In combination, an oscillation generator comprising a vibratoryelement, an electron dischargedevice input and output circuits connectedto said device, means including a drive coil coupled to said outputcircuit for supplying a driving force to vibrate said element, meansincluding -a pick-up coil for controlling the frequency of current insaid output circuit in accordance with the frequency of vibration ofsaid element, and means for varying the frequency of vibration of saidelement, said last-named means comprising a phase-shifting circuitconnected between said pick-up coil and said input circuit, a phaseshifting circuit connected across said output circuit, and means foradjustably connecting said drive coil across saidlast phase-shiftingcircuit, whereby the phase angle between said driving force and saidelement may be varied.

7. In combination, an oscillation generator comprising a vibratoryelement, an electron discharge device, input and output circuitsconnected to said device, means including a drive coil coupled to saidoutput circuit for supplying a driving force to vibrate said element,means including a pick-up coil for controlling the frequency of currentin said output circuit in accordance with the frequency of vibration ofsaid element, and means for varying the frequency of vibration of saidelement, said last-named means comprising a phase-shifting circuitconnected between said pick-up coil and said input circuit, a capacitorand a first resistance connected in series across said output circuit,said capacitor and first resistance having a common point, a secondresistance connected in shunt to said output circuit, and means forconnecting said drive coil between said common point and a variablepoint on said second resistance, whereby the phase angle between saiddriving force and said element may be varied.

MILTON S. MEAD, JR.

